Effects of reference laser intensity noise in a Fourier transform spectrometer

Many noise sources deteriorate the performance of a Fourier-transform spectrometer. When such an instrument utilizes a reference source such as an HeNe laser to sample the measurement data, it is desirable to determine how the intensity fluctuations of the reference source will affect the final spectrum. This text shows how the intensity noise of the reference laser is transformed at the output of the interferometer, how it translates into a sampling jitter by a simple linear relation under a small bandwidth approximation, and how it is manifested in the final spectrum as a noise floor. Analytical and simulation results show that for an instrument designed for optical communication applications, the intensity noise of the reference laser may be one of the dominant noise source limiting the overall performance of the instrument.     

Suppression of intensity noise of a laser-diode-pumped single-frequency Nd:YVO4 laser by optoelectronic control

The intensity-noise reduction of a laser-diode-pumped single-frequency ring Nd:YVO4 laser when different optoelectronic control systems are used is theoretically and experimentally investigated. It has been demonstrated that combining two techniques, optoelectronic feedback control of the drive current of the pump laser diode and feed-forward control of the output laser beam, is a good way to significantly suppress the intensity noise of a laser at low frequency.     

Real-time adaptive image impulse noise suppression

In this paper, a new intelligent hardware module suitable for the computation of an adaptive median filter is presented for the first time. The function of the proposed circuit is to detect the existence of impulse noise in an image neighborhood and apply the operator of the median filter only when it is necessary. The noise detection procedure can be controlled so that a range of pixel values is considered as impulse noise. In this way, the blurring of the image in process is avoided, and the integrity of edge and detail information is preserved. Experimental results with real images demonstrate the improved performance. The proposed digital hardware structure is capable to process gray-scale images of 8-bit resolution and is fully pipelined, whereas parallel processing is used in order to minimize computational time. In the presented design, a 3/spl times/3 or 5/spl times/5 pixel image neighborhood can be selected for the computation of the filter output. However, the system can be easily expanded to accommodate windows of larger sizes. The proposed digital structure was designed, compiled and simulated using the MAX+PLUS II Programmable Logic Development System by Altera Corporation. For the implementation of the system the EPF10K200SRC240-1 field-programmable gate array device of the FLEX10KE device family is utilized, and it can be used in industrial imaging applications where fast processing is required. The typical clock frequency is 65 MHz.     

Low-frequency intensity noise in semiconductor lasers

The low-frequency intensity noise at 25 MHz of a Fabry-Perot semiconductor laser is measured as a function of injection current. All the measurements are taken at room temperature and the laser is operated with a commercial current source (the conditions under which laser diodes are often used). At the highest injection current of twice threshold, the intensity noise is 5.5 dB above the shot-noise limit. When the longitudinal side mode suppression of the laser is 20 dB or larger, the intensity noise is modeled adequately by an expression derived from the single-mode, small-signal, linearized, semiclassical rate equations. All the parameters used in the theory are derived or referenced.     

Demodulation of intensity and shot noise in the optical heterodyne detection of laser interferometers for gravitational waves

Demodulation of intensity noise in the optical heterodyne detector is analyzed for application in interferometric gravitational-wave detectors. The correlation function and the power spectral density of the demodulated intensity noise are derived, taking into account the effect of bandpass filtering at the photodiode and an arbitrary demodulation waveform. The analysis includes demodulation of the rf-modulated shot noise as a special case of the intensity noise. For shot-noise-limited detection, the signal-to-noise ratio is found as a function of the modulation parameters, and the optimization of the signal-to-noise ratio with respect to the demodulation phase is described.     

Quantum noise of laser diodes

Abstract

Single-mode laser theory for semiconductor lasers predicts sub-Poissonian light generation for a laser quietly driven far above threshold. Experiments have shown however that only few laser diodes exhibits such reduced intensity noise. We present a review of different mechanisms that have been proposed to explain the excess noise observed in semiconductor lasers, including imperfect anticorrelations in multimode lasers, and Petermann excess noise factor in single-mode lasers.     

Multimode analysis of relative intensity noise associated with intensity modulation of semiconductor lasers

This paper analyzes the relative intensity noise (RIN) associated with intensity modulation (IM) of multilongitudinal mode semiconductor lasers. We considered two states of operation; namely, stable single-mode and multimode hopping that correspond to weak and strong asymmetric gain suppression. The former is typically considered by AlGaAs lasers, whereas the latter is considered by InGaAsP lasers. We introduce comparison of the influence of the modulation parameters on both the total and modal RIN between both lasers. We show that the IM response and 2HD display a peak at the relaxation frequency. The multimode hopping in the InGaAsP laser induces levels of signal distortion much higher than those in the AlGaAs laser in the regime of low modulation frequencies. Under weak modulation, the total and modal RINs of both the lasers vary little from those of the non-modulated lasers. The deep modulation releases the multimode hopping phenomenon in the InGaAsP laser and suppresses the mode-hopping peak in the RIN spectrum.     

抑制噪声的宽带碲基掺铒光纤放大器性能分析

设计了一个带光隔离器的复合型宽带碲基掺铒光纤放大器（EDTFA）,通过对该结构模型下的速率方程和光功率传输方程组的数值模拟,理论研究了EDTFA在插入光隔离器后的性能变化。研究表明,通过插入光隔离器抑制反向传输的放大自发辐射（ASE）噪声,可以有效地改善宽带EDTFA的信号增益和噪声特性。在光纤激活长度240cm、铒掺杂浓度2000ppm和前向泵浦功率200mW下,光隔离器插入在最佳位置处时,1540nm～1600nm波段内16路信道小信号增益分别提高了0.8dB～5．9dB,噪声系数降低了0.2dB～2．2dB。研究结果对于新型宽带EDTFA应用于WDM通信系统中具有理论指导意义。     

Relative intensity noise of unipolar intersubband semiconductor lasers

Abstract

An analysis is undertaken of the relative intensity noise (RIN) properties of intersubband semiconductor lasers. It is shown how the characteristic time constants of the laser dynamics determine the RIN of a variety of structures.     

Properties of diode lasers with intensity noise control

The effect of the reduction of intensity noise by electronic feedback on the frequency stability of a GaAlAs laser is investigated. Only a small increase (approximately 30%) in stability was observed because the intensity and frequency noise were found to be not completely correlated. The technique described can also be used to estimate the coherence function gamma2 between the various noise properties of the laser's emission without using a correlator.     

Time-frequency-based non-stationary interference suppression for noise radar systems

The problem addressed here is non-stationary interference suppression in noise radar systems. Towards this aim, two linear time ?frequency (TF) transforms, short-time Fourier transform and local polynomial Fourier transform are used as a means of signal representation and filtering. The noise radar return signal is a wideband random signal occupying the whole TF plane, whereas the interference signal is well concentrated in the TF plane. This implies that the filtering of the received signal can be performed by using a binary mask to excise only a portion of the TF plane corrupted by the interference. Simulations carried out on the radar return signal corrupted by extremely strong non-stationary interferences confirm the effectiveness of the proposed method.     

Strong suppression of electrical noise in bilayer graphene nanodevices

Low-frequency 1/f noise is ubiquitous and dominates the signal-to-noise performance in nanodevices. Here we investigate the noise characteristics of single-layer and bilayer graphene nanodevices and uncover an unexpected 1/f noise behavior for bilayer devices. Graphene is a single layer of graphite, where carbon atoms form a two-dimensional (2D) honeycomb lattice. Despite the similar composition, bilayer graphene (two graphene monolayers stacked in the natural graphite order) is a distinct 2D system with a different band structure and electrical properties. 1,2In graphene monolayers, the 1/f noise is found to follow Hooge's empirical relation with a noise parameter comparable to that of bulk semiconductors. However, this 1/f noise is strongly suppressed in bilayer graphene devices and exhibits an unusual dependence on the carrier density, different from most other materials. The unexpected noise behavior in graphene bilayers is associated with its unique band structure that varies with the charge distribution among the two layers, resulting in an effective screening of potential fluctuations due to external impurity charges. The findings here point to exciting opportunities for graphene bilayers in low-noise applications.     

Speckle suppression in scanning laser display

The theory of speckle noise in a scanning beam is presented. The general formulas for the calculation of speckle contrast, which apply to any scanning display, are obtained. It is shown that the main requirement for successful speckle suppression in a scanning display is a narrow autocorrelation peak and low sidelobe level in the autocorrelation function of the complex amplitude distribution across a scanning light beam. The simple formulas for speckle contrast for a beam with a narrow autocorrelation function peak were obtained. It was shown that application of a diffractive optical element (DOE) with a Barker code phase shape could use only natural display scanning motion for speckle suppression. DOE with a Barker code phase shape has a small size and may be deposited on the light modulator inside the depth of the focus of the reflected beam area, and therefore, it does not need an additional image plane and complicated relay optics.     

Noise of He-Cd laser and its suppression

Linear relations have been observed between the reciprocal of the noise ratio and the reciprocal of the output power of the He-Cd laser as well as between the noise ratio and the net gain within the laser cavity. By a feedback method with an acoustooptic modulator in the laser cavity at the Brewster angle for low optical loss, a stable laser with high power output and low noise has been obtained. The noise ratio and the output power are 0.8% and 33 mW, respectively, at 4416 A     

Speckle noise in laser bar-code-scanner systems

We present a theoretical model and its experimental verification for speckle-induced noise in laser-based bar-code-scanner systems. We measured the dependence of the signal-to-speckle-noise ratio on distance, spot size, and detector size. Analyses of the power spectra of both the speckle noise and of the measured surface profiles of different substrates suggest that the paper surface granularity can be approximated by a white Gaussian noise process, thus confirming the assumption of the theoretical model.